KATE, the Duchess of Cambridge has been left to celebrate her 37th birthday without her husband as Prince William attended an event with an air ambulance crew.

The Duchess of Cambridge is today celebrating her birthday “privately” according to Kensington Palace, while her husband attended an event at the Royal London Hospital marking 30 years of London’s Air Ambulance. The Duke aptly flew to the hospital by helicopter landing on the rooftop. There, he met pilots, clinicians and paramedics to hear about the challenges they face working in London including their response to the Grenfell fire and the 7/7 London bombings.

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Looking relaxed, he chatted to paramedics about how the charity provides for the team’s mental health after challenging incidents.

The prince also watched a demonstration of a thoracotomy - a proceedure often used to treat victims of stabbing injuries.

Having worked for the East Anglian Air Ambulance from 2015-2017 when the family was living in Norfolk, it is a cause close to his heart.

Prince William landed on the roof of the hospital by helicopter (Image: Getty )

The Duke, pictured arriving in the cockpit (Image: Getty )

Before his work for the air ambulance he also served nearly five years as a helicopter pilot with the RAF’s Search and Rescue Force.

Kensington Palace announced on Twitter that Prince William has become patron of the London Air Ambulance’s saving lives campaign “which aims to highlight the charity’s vital work in delivering life-saving treatment across London, and raise support for the development of new facilities".

His visit comes after it was revealed that he clocked up less official royal engagements last year than his father Prince Charles or his aunt Princess Anne who was the hardest working royal with 518 total engagements.

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He watched a thoracotomy demonstration (Image: London's Air Ambulance Charity)

Prince William performed 220 engagements while his brother Prince Harry did 193, according to figures compiled by Tim O’Donovan, a retired insurance broker who has been calculating the list of engagements for the past 37 years.

The Prince will still have time to mark Kate’s birthday after the Air Ambulance event.

The couple thought to be celebrating quietly with their three children at home in Kensington Palace.

Kate turns 37 today (Image: Getty)

Prince William on his final shift with the East Anglian Air Ambulance (Image: Getty)

It is Kate’s her first birthday as a mother-of-three, and although Prince George is expected to attend school during the day, it is assumed the Duchess of Cambridge is spending her big day with all three children.

Kate’s family, including parents Carole and Michael and siblings James and Pippa, may also join the celebrations with their significant others.

On her birthday last year, the Duchess was heavily pregnant with Prince Louis and the family spent a quiet day at their Kensington Palace home.